Recently, more stringent standards have been proposed which seek to limit the level at which power-consuming devices are permitted to introduce noise, power frequency harmonics, or other disturbance onto the AC power line as a result of their operation. The IEC555-2 and IEC555-3 standards were initiated by the IEC, amended, approved, renumbered as EN60555-2 and EN60555-3, and implemented by CENELEC for use by members of the European Union. The EN60555-2 and EN60555-3 standards have more recently been updated and renumbered as EN61000-3-2 and EN61000-3-3. All of these standards will be collectively called the "IEC555 Standard". The IEC555 Standard regulates the effects of the power draw of the load upon the current and voltage characteristics of the AC power line. Herein, the term disturbance is used to refer to any of the above-identified effects, as well as one or more of the following: increased average or root mean squared (rms) line current, reduced power factor, and distortions of the AC line voltage, including flattening of the peak voltage levels and/or periodic changes that would cause a visual flicker of the lighting. These specific types of disturbance to the AC power line voltage and current are known to be present when existing electrical equipment such as computers, audio-visual reproduction and recording equipment, lamp dimmers, motor drives, electronic ballast lights, and photocopying equipment, among others, are powered from an AC power line source.
Prior to the invention disclosed herein, no system known to the inventors was capable of supplying variable power from an AC power line source to a resistive AC load that could also meet the requirements of the IEC555 Standard for minimizing disturbance to the AC power line voltage and current. A system in use prior to the proposal of the IEC555 Standard is known as thyristor AC phase control. An example of such system is described in U.S. Pat. No. 5,373,224 to Rabier ("the Rabier Patent"). A further example is shown in FIG. 6A of an extended range full wave phase control circuit. In these types of systems, which may commonly be used in photocopiers, lamp dimmers, heater controls and cooking appliances, a triac placed between the AC power line and the load is "fired", i.e. switched on, at some delay relative to the start of each half cycle of the AC power line voltage, such that power is supplied to the load during only a predetermined portion of each half cycle of the AC line voltage. In that way, the triac controls delivery of power to the load in accordance with the relative proportion of each AC power cycle in which the triac is switched on.
However, while prior triac-controlled switching power supplies are capable of meeting the voltage fluctuating requirements of the IEC555-3 (now EN61000-3-3) standard, they are incapable of meeting the harmonic requirements of IEC555-2 (now EN61000-3-2) standard because the voltage and current waveforms supplied to the load are not sinusoidal at substantially a single frequency.
An alternate method of controlling the triac-controlled power supply is to switch it on for several cycles and then off for several cycles, resulting in a very slow modulation frequency. An example of such a system is shown in FIG. 6B employing a solid state relay with zero crossing turn on. The circuit of FIG. 6B is similar to that of FIG. 6A, however, it can be controlled by a computer. Accordingly, the triac can be on for extended periods of time and is slowly modulated on and off to control the average power. Although this method, which is currently in use for many heater type applications in the electronics industry such as photocopiers meets the harmonic distortion requirements, it does not meet the fluctuating voltage restrictions and causes visual flicker to the lighting. Accordingly, it fails the standard IEC555-3 (now EN6100-3-3).
FIG. 1 contains a set of waveforms plotted versus time for 1) the voltage Vs and current Is on an AC power line with minimal disturbance present; 2) the voltage VL and current IL as supplied to a load through a prior triac-controlled supply; and 3) an example of the AC power line voltage Vs1 and current Is1 under disturbed conditions, i.e. as voltage and current being supplied to the load by the triac-controlled power supply. As shown in FIG. 1, irregular, non-sinusoidal voltage and current are supplied to the load as waveforms VL and IL The irregular current draw, in turn, disturbs the AC power line characteristics, resulting in the disturbed voltage and current waveforms Vs1 and Is1.
Like the prior thyristor AC phase control system, the present invention is designed to supply power to AC loads used in photocopiers, lamp dimmers, heater controls, cooking appliances, and many other types of equipment which draw sinusoidal AC power. The present invention operates to minimize the level of disturbance to the AC power line while providing variable AC power to a load and meeting the requirements of the IEC555 standard.
Accordingly, it is an object of the present invention to provide a switching power supply which delivers a controlled amount of power to an AC load from an AC power line while minimizing disturbance to the voltage and current which are carried by the AC power line.
A further object of the invention is to provide a switching power supply which draws power at timed cycle intervals from a source while delivering a continuous sinusoidal voltage and current to the load.
Another object of the invention is to provide a switching power supply which draws power at timed cycle intervals from a source while maintaining substantially sinusoidal voltage and current waveforms at the source.
Another object of the invention is to provide a switching power supply which provides power to an AC load at a modulation frequency substantially higher than the AC source frequency so that there will be no fluctuating distortion on the source that would create a visible flicker.
Still another object of the invention is to provide a more reliable switching power supply which contains few components.
A still further object of the invention is to provide a low-cost switching power supply for use with a full spectrum of consumer and business equipment.